1. Field of the Invention
This invention relates to a method for forming single silicon crystals by laser crystallization.
2. Description of Related Art
As the demand for higher performance two-dimensional arrays used in devices such as displays and scanners increases, the requirements for higher performance thin film transistors (TFTs) to integrate high speed electronic circuits with the two-dimensional arrays also increase. However, because conventional TFTs are manufactured using polycrystalline silicon thin films having small grain sizes, the performance of conventional TFTs is limited by undesirable low field-effect mobilities inherent in these polycrystalline silicon thin films.
Field-effect mobilities are reduced by grain boundaries. Thus, if the polycrystalline silicon film contains large grain sizes, then the number of grain boundaries per unit area would be reduced and the field-effect mobility would be improved. Accordingly, many efforts have been advanced to increase field-effect mobilities by producing polycrystalline silicon thin films having large crystal grain sizes so that greater performance polycrystalline silicon TFTs may be obtained.
Prior efforts to increase polycrystalline silicon grain sizes have relied on solid phase crystallization in which an entire assembly (i.e., the substrate and deposited films) is heated at high temperatures for many hours to encourage large crystal growth. However, solid phase crystallization has several drawbacks: 1) throughput is low because a large amount of time is required to form large crystal grains; and 2) glass substrates are subjected to high temperatures causing many adverse effects such as warping and bending of the glass substrates.
Besides having small grain sizes, the polycrystalline silicon films formed by prior processes also have a problem of varying grain densities. Since polycrystalline silicon grains are formed randomly, there is no control of the number of grains formed or the distribution of grain sizes. In fact, many polycrystalline silicon films have greatly varying grain sizes. The performance of TFTs formed in these polycrystalline silicon films strongly depends on the locations of the devices since grain size and density vary greatly over the substrate area. Thus, dependable electronic circuitry cannot be realized even though the average polycrystalline silicon grain size is increased.